jpadc
Well-known member
fooljoe said:Hmmm, does this mean the Leaf is violating the J1772 protocol? I thought the car was expected to adjust to a changing pilot signal in real time. For example if using the "Hydra" or other dual-headed EVSE one might start charging at 30 amps but then have that cut in half to 15 amps without interrupting the charge if a second car plugs in, and similarly, if the second car unplugs the car should ramp back up. This isn't a safety concern for the second case of course, but it's very troubling that the Leaf would apparently keep charging at 28 amps even when the pilot drops to 12 amps.
Well, an EVSE can't "know" what other things are on the same electrical circuit that it is on so it can't possibly adjust allowable amperage draws based on other things on the same circuit (including other EVSEs).
It can detect a voltage change (i.e. one power leg being dropped), but the amperage setting are programmed into the software of the EVSE (or are adjustable like on the Juicebox). An EVSE only reports and delivers the amperage the car has requested up to its programmed max setting. I don't know about Open EVSE, but the Juicebox has a separate adjusters for the max amp draw allowed for 120 verses 240 volt input. The EVSE encodes that max setting in the pilot duty cycle (ie, signal frequency) and that is how it conveys maximum allowed amperage information to the car. The pilot signal does not encode the voltage level (i.e., 120 versus 240) as that is directly known to the vehicle depending on the presence of current on the AC Line 1 and AC Line 2 pins of the J1772 plug.
So if the car is told via the pilot signal, when it is first plugged in that the EVSE is programmed to deliver up 30 amps, the car may ask for up to that. I don't think there is anything in the specification of the protocol that requires the car's charger to keep monitoring the pilot signal's duty continuously, only its voltage swings (to know when to start and stop charging). It is required to check the duty cycle when it when its first plugged in. Even a drop in power (i.e., a power failure) does not require a reread of the pilot signal's frequency as it should come back up the same as it was before the power failure. It is required to set the voltage back to -9/+12 volts so the car will delay the restarting of charging for a short while after power is restored. Read here if you care about details of the protocol: https://code.google.com/p/open-evse/wiki/J1772Basics.
Long and short, if you want to be safe, pull the J1772 plug out of the vehicle (which will be detected and registered on the proximity signal by the car) before ever changing any voltages to the EVSE. That way the car will be sure to get the proper information when plugged back in and not try to draw more ams than it should.
P.S. I think the "two headed" Hydra actually limits charging by programming to 15 amps to each head so it can be ready for a second car. I don't think it adjusts amps available based on one or two cars being plugged in. - But you have me curious.